1. Field of the Invention
The present invention relates to a motor vehicle steering system which is adapted to drive a steering mechanism on the basis of the operation of an operation member such as a steering wheel for turning steerable vehicle wheels.
2. Description of Related Art
A steering system (so-called “steer-by-wire system”) for a motor vehicle has been proposed, which has no mechanical coupling between a steering wheel and a steering mechanism for turning steerable vehicle wheels, and is adapted to detect the direction and amount of the operation of the steering wheel and apply a driving force to the steering mechanism from a steering actuator such as an electric motor on the basis of the results of the detection.
With this arrangement, a ratio (gear ratio) between the turning amount of the steerable vehicle wheels and the operation amount of the steering wheel can flexibly be changed according to the traveling state of the motor vehicle for improvement of the maneuverability of the motor vehicle. In addition, the aforesaid arrangement is advantageous in that the upward thrust of the steering wheel can be prevented at collision of the motor vehicle and in that the steering wheel can more flexibly be located.
FIG. 3 is a block diagram illustrating the electrical construction of the steer-by-wire system. Detection signals such as indicative of the direction and amount of the operation of the steering wheel are inputted to a main electronic control unit (ECU) 101. A principal steering motor 105 and an auxiliary steering motor 106 are connected to the steering mechanism, and a counter force motor 107 is connected to the steering wheel. The principal steering motor 105 and the auxiliary steering motor 106 are each adapted to apply the driving force to the steering mechanism for turning the steerable vehicle wheels. The auxiliary steering motor 106 is operative to apply the driving force to the steering mechanism when the principal steering motor 105 and/or its control system malfunction. The counter force motor 107 applies a counter force to the steering wheel according to a reaction force from a road surface.
The principal steering motor 105 is controlled to be driven by a principal steering electronic control unit 102, and the auxiliary steering motor 106 is controlled to be driven by an auxiliary steering electronic control unit 103. The counter force motor 107 is controlled to be driven by a counter force electronic control unit 104. The principal steering electronic control unit 102, the auxiliary steering electronic control unit 103 and the counter force electronic control unit 104 are comprehensively controlled by the main electronic control unit 101.
Detection signals are inputted to the main electronic control unit 101 from an operation angle sensor 111 for detecting the operation angle of the steering wheel, a torque sensor 112 for detecting an operation torque applied to the steering wheel, a steering angle sensor 113 for detecting the turning angle of the steerable vehicle wheels and a vehicle speed sensor 114 for detecting the speed of the motor vehicle (vehicle speed).
The main electronic control unit 101 controls the principal steering electronic control unit 102, the auxiliary steering electronic control unit 103 and the counter force electronic control unit 104 on the basis of the detection signals from the sensors 111 to 114. That is, the main electronic control unit 101 transmits necessary ones of the detection signals from the sensors 111 to 114 to the principal steering electronic control unit 102, the auxiliary steering electronic control unit 103 and the counter force electronic control unit 104, and generates control commands for controlling the electronic control units 102 to 104.
Thus, the steering force is properly transmitted from the principal steering motor 105 to the steering mechanism according to the operation of the steering wheel, and the counter force is properly transmitted from the counter force motor 107 to the steering wheel according to the reaction force from the road surface. If the principal steering electronic control unit 102, the principal steering motor 105 or the like malfunctions, the auxiliary steering electronic control unit 103 properly controls the auxiliary steering motor 106 upon reception of a control command from the main electronic control unit 101.
With the aforesaid arrangement, however, the detection signals from the sensors 111 to 114 which detect the steering conditions of the motor vehicle are inputted only to the main electronic control unit 101, but not inputted directly to the principal steering electronic control unit 102, the auxiliary steering electronic control unit 103 and the counter force electronic control unit 104.
Therefore, the detection signals from the sensors 111 to 114 are not transmitted to the electronic control units 102, 103, 104, if the main electronic control unit 101 malfunctions. This makes it impossible to properly control the principal steering motor 105, the auxiliary steering motor 106 and the counter force motor 107.